Idk, the internet, smoking rates, queer issues, maga, americans identifying as socialists, mass surveillance, etc.

Marty would be showing up in the 90s with a mustache and crop top, confused as to why everything is smoky as hell and why so many people are calling him homophobic slurs, as he has to figure out how to get onto and use the internet. Also a lot of his slang will be incomprehensible to the people of the 90s